Transmitting apparatus for communication systems



June 23, 1942. 1.. D. WHITELOCK ETAL 2,237,655

TRANSMITTING APPARATUS FOR COMMUNICATION SYSTEMS Filed April 22, 1941 dm m Y v L T fibmfiimfimb w R NQRKS KQMQ W m m Y J MM m mm a H $14 m? m NV w x R b QM Km: a, fiww M MN his mfifi Patented June 23, 1942 UNITEDSTATE TRANSMITTING APPARATUS FOR COMMUNICATION SYSTEMS Leland D.Whitelock, Wilkinsburg, and Ralph K. Crooks; Sharpsburg, Pa., assig'norsto The Union Switch & Signal Company, Swissvale, Pa., a

corporation of Pennsylvania Application April 22, 1941, Serial No.389,762

10 Claims.

- Our invention relates to transmitting apparatus for communicationsystems, and more particularly to such apparatus for telephone systemsus ing suppressed carrier transmission.

Communication systems which provide carrier telephony are in general useand in many instances suppressed carrier single side band transmissionis employed. Such communication systems are, for example, providedfor'railway trainswhere both telephoning and signaling between a waysidestation and a station on a train, or between two stations located one oneach of two trains, or between two spaced stations on the same train arerequired. The telephoning is preferably effected by suppressed carriersingle side band transmission while signaling and calling is effected bythe transmission of the equivalent of the carrier modulated by a singlepreselected frequency within the voice frequency band. In such system,most satisfactory operation of the transmitting apparatus and of thesystem in general, is obtained when the output of the transmittingapparatus is of substantially the same energy level for the signalingand. calling currents as for the telephone current, becauseover-energization of the usual .power amplifier and other equipment justahead of the transmitting circuit as well as a poor wave shape andharmonics may result if a high energy level of the signaling and callingcurrents is used.

In view of the circumstances recited above, a feature of our inventionis the provision .of novel and improved transmitting apparatus forcommunication systems. I

Another feature of our invention is the provision in transmittingapparatus for communication systems using suppressed carrier single sideband transmission of novel and improved means for providing a signalingor a calling current.

Again, a'feature of our invention is the provision of novel control andswitching means for transmitting apparatus of the type here involvedwherewith either a condition for delivering a suppressed carrier singleside band telephone current our invention, we attain by including in thetransmary winding of a modulator output transformer disposed in apush-pull arrangement. The grid circuits of the two tubes are of thebalanced type to which a carrier generator oroscillator and a microphonecircuit are connected in such a manner that the carrier is modulated bythe voice frequen y produced in the microphone, the carrier issuppressed and the side bands resulting from such modulation appear inthe secondary winding of the modulator output transformer. Thissecondary winding is connected to the associated transmitting circuitpreferably through a filter proportioned to pass a' particular one ofthe side bands only. If further amplification of the selected side bandis desired, the necessary stages of amplification are interposed betweenthe filter and the transmitting circuit.

' The control and switching means includes a special shunt-circuit forthe modulator and circuit switching devices preferably in the form ofrelays. One such switching device or relay by its position controlst-l.e frequency of the carrier current delivered by the generator of theapparatus, the eifective or inefiective condition of at least one of thetubes of the modulator and theopen or closed condition of the shuntcircuit. At

a first position of such device or relay,.the car-- rier generator isconditioned to deliver-a carrier current of a preselected frequencysuitable for modulation byvoice frequencies,- both tubes of themodulator are made eifective and the shunt circuit is open with theresult that the transmitting apparatus is conditioned for supplying asuppressed carrier single side band telephone .current, the energy levelof such current being predetermined by the proportioning of the severalparts. At a second position of such switching device or relay, thegenerator is adjusted for delivering a carrier current of a frequencypreselected within the side band of the transmitted telephone current,one tube of the modulator is rendered ineffective and the shunt circuitis ing current. Proper proportioning of the shunt circuit causes thatportion of the signaling current delivered to thefilter, and in turn tothe transmitting circuit, to be of an energy level of the same order asthe energy level of the tele-' phone current. That is, this signalingcurrent supplied by the carrier generator is preselected as to itsfrequency so as to fall within the band of the transmitted telephonecurrent, and a sufficient portion of the signaling current is shuntedaway from the output of the modulator by the special shunt circuit so asto leave the energy level of the portion of the signaling currentapplied to the filter to be of the same order as the energy level of thetelephone current.

A second switching device or relay is preferably used to govern theconnections of the current sources to the apparatus and to govern theeffective or ineffective condition of the microphone circuit. At a firstposition. of such second device or relay, the sources of current aredisconnected from the plate circuits of the electron tubes of theapparatus, and the apparatus is retained in a standby condition. Suchstandby condition isdesirable in systems of the type here contemplatedbecause the receiving apparatuses at the different stations of thesystem are energized and in an active condition ready to receive energyfrom the transmitting circuit, and hence it is desirable that thetransmitting apparatus at each station of the system be retained in anineffective standby condition. sition of this second device or relay,power is applied to the plate circuits of the several electron tubes andthe transmitting apparatus is made active and ready for transmission ofcurrent. The microphone circuit is also rendered efiective at thissecond position of the second device. Preferably, the conditions of theapparatus as effected by the switching devices are interlocked so thatsignaling current may not be delivered by the carrier generator whiletelephone communication is going on.

We shall describe one-form of apparatus embodying our invention, andshall then point out the novel features thereof in claims.

The accompanying drawing is a diagrammatic view showing one form oftransmitting apparatus embodying our invention, and which apparatusisparticularly useful for a communication system for railway trains, butis in no manner limited thereto in its use.

Referring'to the drawing, the transmitting apparatus comprises amicrophone M, a carrier generator G, a balanced modulator BM, a bandpass filter BPF, an amplifier AM, a shunt circuit SC, proper sources ofcurrent and switching means, the latter of which will be more'fullydescribed hereinafter.

Referring first to the balanced modulator BM, it includes two electrontubes 4 and 5 which, as here shown, are of the indirectly heated cathodetype, the filaments of'these tubes being heated by any convenient means,such as a battery 6. The control grids I and 8 of tubes 4 and 5,respectively, are connected to balanced grid circuits of any convenienttype, and in the instant application the grids I and 8 are connected tothe respective outside terminals of a first resistor 9, the centerterminal of which is connected to the cathodes Illand II of the tubesthrough a second resistor I2 and a biasing unit I3 of the potentiometertype. A ground electrode I4 is connected at the junction of resistor Ata second po- 29, wires 28 and 25, front contact 24 of a relay DR to belater described, resistor 23 and ground electrodes 22 and 3| to thelower terminal of microphone -M, a by-pass condenser 36 being preferablyconnected between the ground electrode 3| and primary winding 30. Itfollows that at such time as relay DR is picked up, closing frontcontact 24 and current flows in the plate circuits of tubes 4 and 5, aswill be shortly explained, the voltage drop across resistor 23 serves toenergize the microphone circuit with the result that electromotiveforces created by speaking into microphone M cause electromotive forcesof corresponding frequencies to be applied across resistor 9 of the gridcircuits of tubes 4 and 5, but that when relay DR is released to openthe plate circuits of the modulator tubes, the microphone circuit ismade ineffective.

Modulator tubes 4 and 5 are provided with plate circuits having apush-pull arrangement and which circuits are energized from anyconvenient source of current, which in the present embodiment of theinvention consists of a transformer TI and rectifiers RI and R2. Aprimary winding I5 of transformer TI is connected to the BX and CKterminals of a suitable source of alternating current, such as agenerator not shown, while a secondary winding I6 of transformer TIsupplies through the rectifiers RI and R2 unidirectional current to theplate circuits. To be explicit, the plate circuit for tube 4 can betraced from the top outside terminal of secondary winding I6 oftransformer TI over rectifier RI in its low resistance direction to wireII, or from the lower outside terminal of secondary winding I6 overrectifier R2 in its low resistance direction to wire I1, and thencethrough impedance I8, resistor I9, center terminal of primary winding 20of an output transformer OT, top half portion of primary winding 20,plate 2I of tube 4, intervening tube space to cathode III of the tube,biasing unit I3, ground electrode I4, ground electrode 22, resistor 23,front contact 24 of relay DR and wire 25 to the center terminal ofsecondary winding I6 of transformer TI. Similarly, the plate circuit fortube 5 is the same as the circuit for tube 4 up to the center terminalof primary winding 20 of output transformer OT, and thence through thelower half portion of primary winding 20, wire 53, back contact 54 ofrelay DRI, also to be later described, wire 55, plate 26 of tube 5,intervening tube space to cathode II of the tube, biasing unit I3,ground electrode I4, and thence to the center terminal of secondarywinding I6 of transformer TI over ,the same path as traced for the platecircuit of tube 4.

Carrier generator G is connected to the grid circuits of tubes 4 and 5by being connected across resistor I2, one output terminal of generatorG being connected to one terminal of resistor I2 over wire 3| and theother output terminal of generator G being connected to the otherterminal of resistor I2 over wire 32 and ground electrodes 33 and I 4.The type of generator G is immaterial and it is shown conventionally forthe sake of simplicity. In the present embodiment of the invention it iscontemplated that generator G is of the electron tube oscillator typeincluding an oscillating circuitthe physical elements of which determinethe frequency of the current delivered by the generator. Looking atgenerator G, a condenser 34 is interposed in the oscillating circuit ofthe generator over a back contact 35 of relay DRI. When relay DRI isreleased closing back contact 35 and condenser 34 is included in theoscillating circuit of the generator, the generator delivers current ofa preselected carrier frequency and when relay erator and that thecarrier is raised 1050 cycles to 6750 cycles per second when condenser34 is disconnected from the oscillating circuit. Since 1050 cycles persecond falls within the voice frequency range, it is to be seen that thecarrier current is raised in frequency by a preselected voice frequencywhen condenser 34 is disconnected. It is clear that the same results inadjusttuned or oscillating circuit instead of varying the capacity ofthe circuit. Also, it is clear that when the 'band pass filter BPF isadjusted for lower side band transmission, the frequency of the carriermay be decreased to a predetermined frequency, such as 4650 cycles,which is the equiva lent of the lower side band of a carrier of 5700cycles modulated by a 1050 cycle current,

cerned is the fact that generator G is conditioned to deliver a carrierof a preselected frequency when relay DRI is released and isconditioned'to deliver a carrier of a frequency varying from such Ipreselected frequency by a preselected voice frequency when relay DRI ispicked up. It is to be understood of course that our invention is notlimited to the specific frequencies assumed above, and other frequenciescan be used if desired.

Consequently when relay DR is picked up closing front contact 24 tocomplete the plate circuits for tubes 4 and 5, and relay DRI- isreleased closing back contacts 35 and 54, a carrier of a frequency of5700 cycles per second is modulated by the, voice frequency produced inthe microphone circuit, the carrier is balanced out by the pushpullarrangement of the plate circuits of tubes 4 and and the resultant sideband electromotive forces are created in secondar winding. 31 of theoutput transformer CT, the energy level of such electromotive forcesbeing predetermined by the proportion of the parts.

Band pass filter BPF has its input terminals connected to secondarywinding 31 of transformer OT over wires 38 and 39 and is proportioned topass a selected one of the side bands and to suppress the other,. andwe' shall assume that the upper side band is passed by the filter BPFand the lower side band is suppressed. It is to be understood, ofcourse, that the band pass filter may be proportioned to select thelower side band and suppress the upper side band if desired. With filterBPF proportioned to pass the upper side band, it passes a band offrequencies extending from 6200 to 8200 cycles per second, on the as-,sumption that the voice frequency band extends from 500 to 2500 cyclesand the carrier is 5700 cycles.

The output terminals of filter BPF are connected by wires 40 and 4| tothe input terminals of an amplifier AM whose output circuit is coupledto a transmitting circuit TC through the medium of a couplingtransformer T3. Amplifier AM may be of any one of several well-knowntypes and is shown conventionally for simplicity. Amplifier AM would bepreferably of the electron tube type, the plate circuit of whichincludes the primary winding 42 of the coupling transformer T3 whosesecondary winding 43 is connected across wires 44 and 45 of thetransmitting circuit TC leading to a remote associated station notshown. Preferably another secondary winding 46 of transformer T3 isconnected to an indication lamp 41 which lamp is thus illuminated as anindication to the operator of the transmitting apparatus that current isbeing supplied to the transmitting ing the frequency of the output ofthe generator 3 can be effected by varying the inductance ofthe,

The significant point as far as our inventionis con-' circuit.

A transformer T4and rectifiers R3 and R4 serve as a source of energy forthe plate circuit of amplifierAM, primary winding 48 of transformer T4being connected to the BX and CK terminals of the source of alternatingcurrent over an obvious circuit including front contact 49 of relay DR.Unidirectional current is supplied to the plate circuit of amplifier AMfrom secondary winding 50 of transformer T4 by a circuit extending fromthe outside terminals of the secondary winding 50 through eitherrectifier R3 or R4, as the case may be, to a wire 5| leading to primarywinding 42 of transformer T3, the other terminal of which winding isconnected to the amplifier, while the other side of the plate circuitextends from another terminal of the amplifier to the center terminal ofsecondary winding 50 overwire 52. The turn ratio of transformer T4 isproportioned, of course, so as to supply current of the proper voltagefor the plate circuit of the amplifier AM. Hence relay DR when releasedopening front contact 49, causes amplifier AM to be deenergized and thecoupling between filter BPF and the transmitting circuit TO to beineffective, but when relay DR is picked up to close front contact 49,amplifier AM is energized and the coupling between filter BPF and thetransmitting circuit is made effective. Relay DR therefore controls atits front contacts 24 and 49 the energization of the plate circuits ofthe tubes 4 and 5 of the modulator and of the amplifier AM. Also, asdescribed hereinbefore, the energization of the microphone circuit iseffected by the voltage drop across resistor 23 placed in the platecircuits of tubes 4 and 5 with the result that relay DR thereforecontrols the effective or ineffective condition of the microphonecircuit.

It should-be pointed out at this point that in the event additionalamplification of the side band energy created in the secondary winding31 of transformer CT is not required, then the amplifier AM may beomitted and the wires 40 from the top terminal of primary winding-20overwire 58, resistor 56, front contact 57 of relay DRI and wire 53 tothe lower terminal of winding 20. The function of the shunt circuit SCwill appear when the operation of the apparatus is described.

The control of the transmitting apparatus is effected by relays DR andDRI which may be in turn governed automatically or manually as desired.In the present embodiment of the invention, relays DR and DRI aregoverned by two manually operable switches FBI and PB2, switch PBI beingpreferably of the biased spring return pushbutton type, and switch PB2being preferably of the non-biased type remaining at either its fullline or its dotted line position except as moved manually from oneposition to the other. The control circuits by which switches FBI andP32 control relays DR and DRI will shortly appear.

In describing the operation of the apparatus, we shall first assume thatboth switches FBI and P32 occupy the positions illustrated by the fulllines in the drawing, o'that the control circuits for relays DR and DR!are open and both relays released. Under this condition of the relays,the plate circuits of modulator tubes 4 and 5 are open at front contact24 of relay DR, and the plate circuit of amplifier AM is open at frontcontact 49 of relay DR with the result the transmitting apparatus isnormally at an inactive standby condition.

In preparation for telephone communication, we shall assume that theoperator first sends a calling current. The operator under suchcircumstances would move the switch PBZ to its dotted line positionwhere a control circuit is formed from terminal B of a suitable sourceof current, such as a battery not shown, over a winding of relay DR,contact 58 of switch PB2 and to terminal C of the same source ofcurrent, and relay DR is energized and picked up closing front contacts24 and 49 and completing the plate circuits for the modulator BM andamplifier AM. A control circuit for relay DRI is also formed fromterminal B over contact 59 of switch PBZ, winding of relay DRI, contact60 of switch FBI and to terminal C, and relay DRI is picked up to openits back contacts 35 and 54 and close its front contact 51. The openingof back contact 35 disconnects condenser 34 from the oscillating circuitof generator G and shifts the carrier frequency from 5700 cycles to 6750cycles per second. The opening of back contact 54 opens the platecircuit of tube 5 of the modulator and that tube is made inactive. Theclosing of front contact 51 completes the shunt circuit SC acrossprimary winding 20 of transformer OT. The carrier current from generatorG is now applied across grid 1 and cathode III of tube 4 only and causesthe plate circuit of tube 4 to have an alternating current component ofthe corresponding carrier frequency, since the normal carriersuppression is prevented due to the inactivity of tube 5. The purpose ofresistor 56 is to provide a means of reducingthe power output of tube 4to the desired level. A variation in the value of resistor 56 alters theimpedance of the output circuit of tube 4, thus effecting a change inthe power delivered by the tube to its load. Thus it is seen that tube 4under these conditions operates as a simple amplifier delivering energyat the signal frequency to the transformer OT. This energy transferredto secondary winding 31 of the output transformenOT as can be traced isof the frequency of 6750 cycles per second and hence falls within thepass band for filter BPF which, as stated hereinbefore, is roportionedto pass the upper side band frequencies extending from 6200 to 8200cycles. Thus the energy now delivered to secondary 'winding 31 serves asa calling current and is passed by filter BPF to amplifier AM and thenis delivered to the transmitting circuit TCthrough transformer T3. Theenergy level of this calling current as delivered to secondary winding31 of transformer OT and in turn to the transmitting circuit; TC canreadily be preselected by the proportioning of resistor 56 of the shuntcircuit. Furthermore, such calling current is supplied by the apparatusto the transmitting circuit as long as switch PBZ remains at its dottedline position and the switch PBI occupies its normal position.

For the purpose of calling, the operator would, of course, retain switchPB2 at its dotted line position only for a short interval. It is to beseen, however, that if it is desired to transmit such current for anindefinite period for the purpose of signaling, such transmission iseffected by retaining the switch PB2 at its second or dotted lineposition. Also, it is clear that by operating switch PB2 between its twopositions in accordance with short and long intervals corresponding to aprearranged'code, code signaling current can be supplied to thetransmitting circuit. It is to be noted that lamp 4'! is illuminated inresponse to the transmission of such current and hence will indicate tothe operator when current is supplied to the transmitting circuit.

Assuming telephone communication is desired after the sending of thecalling current, switch P32 is placed at its normal .position and switchFBI is depressed. With switch PBI depressed,

relay DR is energized over an obvious control.

wind 31 of transformer OT. Filter BPF passes only the upper side band ofsuch modulated current and the current is amplified at amplifier AM andapplied to the transmitting circuit TC for transmission to the remoteassociated station, lamp 4! being illuminated to indicate to theoperator such transmission.

As pointed out hereinbefore, the energy level of the telephone currentcan be predetermined by proper proportioning of the parts. Also, theenergy level of the calling and signaling currents can be made of thesame order as that of the telephone current by proper proportioning ofthe shunt circuit SC. Furthermore, the energy of the telephone currentand of the calling and signaling currents as delivered to filter BPF andin turn to amplifier AM just ahead of the transmitting circuit can bemade of such level that over-energization of these devices, distortedwave shape and harmonics are avoided.

It should be pointed out that, while it is contemplated that the energylevel of the calling and signaling currents is governed by rendering onetube of the modulator inactive and by placing a shunt circuit on themodulator output I 2,287,655 transformer, the desired value ofthecalling and signaling currents may be accomplished by rendering one tubeof the modulator inactive only and the shunt circuit may then not berequired.

It is to be seen, therefore, that we have provided novel and improvedtransmitting apparatus including control and switching me'ans wherewitha calling and signaling current can be readily supplied and-whichcalling and signaling current can be made of an energy levelpredetermined by the proper proportioning of a shunt circuit.

Although we have herein shown and described only one form oftransmitting apparatus for communication systems embodying ourinvention, it is understood that various changes and modifications maybe made therein within the scope of the appended claims withoutdeparting from the spirit and scope of our invention.

Having thus described our invention, what we claim is:

1. In transmitting apparatus for a communication system the combinationcomprising, a

generator operable to deliver a carrier current, a

microphone, a balanced modulator including two electron tubes whoseinput circuits are coupled to said generator and to said microphone toeffect a carrier telephone current in the output circuits of the tubes,control means operable to afirst and a second position, means includingthe first position of said control means to adjust said generator todeliver a carrier current of a preselected frequency suitable for saidtelephone current, means including the second position of said controlmeans to open the output circuit of a selected one of said tubes and to.adjust said generator to deliver a, carrier current ofa frequency equalto said preselected frequency varied by a preselected voice frequencyfor creating a signaling current in the output. circuit of the othertube, and a transmitting circuit coupled to said output circuits fortransmission of either the telephone current or of the signalingcurrent. i

2. In transmitting apparatus for a communication system using suppressedcarrier transmission the combination comprising, a generator operable todeliver a carrier current, a microphone, a balanced modulatorincludingtwo electron tubes whose'input circuits are coupled to said generatorand to said microphone, a control means operable to a first and a secondposition, means governed by said control means at its first position toadjust said' generator to deliver a carrier current of a preselectedfrequency and to connect the output circuits of said tubes in a,push-pull arrangement to provide a suppressed carrier telephone currentin response to voice frequencies created in said microphone, meansgoverned by said control means at its second position to open the outputcircuit of a preselected one of said tubes and to adjust said generatorto deliver a carrier current of a frequency equivalent to saidpreselected frequency shifted by a preselected voice frequency to createa signaling current in the output circuit of the other one of saidtubes,

and means for coupling the'output circuits of said tubes to atransmitting circuit.

3. In transmitting apparatus for a communication system using suppressedcarrier transmission the combination comprising, a generator normallyadjusted to deliver a carrier current of.

a preselected frequency, a microphone, a balanced modulator includingtwo electron tubes whose input circuits are coupled to said generatorand to said microphone to effect a' carrier telephone current in theoutput circuits of the tubes, a'relay deenergized or energized accordingto a first or a second position of a manually operable switch, meanscontrolled by said relay when deenergized to connect the output circuitsof said tubes in a push-pull arrangement to suppress the carrier of saidtelephone current; means controlled by said relay when energized tocondition the generator to deliver a signaling current of a frequencyequivalent to said carrier current modulated by a preselected voicefrequency, to open the output circuit of a selected one of the tubes andto connect a shunt path including resistance across the output circuitof the other tube; and a transmitting circuit coupled to theoutputcircuits of said tubes for transmission of said suppressed carriertelephone current and of said signaling current.

4. In transmitting apparatus for a communication system using suppressedcarrier transmission the combination comprising, a generator normallyadjusted to deliver a carrier current of a preselected frequency, amicrophone, a balanced modulator including two electron tubes whoseinput circuits are coupled to said generator and to said microphone toeffect a carrier telephone current in the output circuits of the tubes,control means operable to a first and a second position, means includingthe first position of said control means to connect the'output circuitsof said tubes in a push-pull arrangement to suppress the carriergof saidtelephone current, a transmitting circuit, means including a filtereffective to pass only a selected .one of the side bands of saidtelephone current to couple said transmittingcircuit to the outputcircuits of said tubes, and means including the second position of saidcontrol means to condition said generator to deliver a signalingcurrent-of a frequency preselected within the band passed by said filterand circuit of'a preselected one telephone current in the outputcircuits of the tubes, control means operable to a first and a v secondposition, means including the first position of said control means toconnect the output circuits of said tubes in a push-pull arrangement to'suppress the carrier of said telephone cur-- rent, means including thesecond position of said control means to render inactive a preselectedone of said tubes and to condition said generator to deliver a signalingcurrent of a frequency equal to said preselected frequency varied by apreselected voice frequency, and a transmitting circuit coupled to saidoutput circuits for trans- 'mission of either said suppressed carriertelephone current or of said signaling current.

, 6. In transmitting apparatus for a communication system usingsuppressed carrier transmis' sion the combination comprising, a balancedmodulator including two electron tubes the plate circuits ofwhich'include respective-portions of an output transformer primarywinding disposed in a push-pull arrangement, means effective at times toapply to the control grids of said tubes a carrier current of apreselected frequency and a-voice frequency current to create in asecondary winding of said output transformer the side bands of saidcarrier modulated by voice frequencies, a transmitting circuit, meansincluding a filter effective to pass a preselected one of said sidebands only to couple said secondary winding to said transmitting circuitfor transmission of such single side band telephone current, meanseffective at other times to apply to the control grids of said tubes asignaling current of a single frequency preselected within the side bandpassed by said filter, and control means operable to open the platecircuit of one of said tubes for transmission of such signaling current.

'7. In transmitting apparatus for a communi-. cation system usingsuppressed carrier transmission the combination comprising, a balancedmodulator including two electron tubes the plate circuits of whichinclude respective portions of an output transformer primary windingdisposed in a push-pull arrangement, means effective at times to applyto the control grids of said tubes a carrier current of a preselectedfrequency and a voice frequency current to create in a secondary windingof said output transformer the side bands .of said carrier modulated byvoice frequencies,

a transmitting circuit, means including a filter effective to pass apreselected one of said side bands only to couple said secondary windingto said transmitting circuit for transmission of such single side bandtelephone current, means effective at other times to apply to thecontrol grids of said tubes a signaling current of a single frequencypreselected within the side band passed by said filter, and controlmeans-operable to render only a preselected one of said tubes effectivefor transmission .for said signaling current.

8. In transmitting apparatus for a communication system using suppressedcarrier transmission the combination comprising, a balanced modulatorincluding two electron tubes the plate circuits of which includerespective portions of an output transformer primary winding disposed ina push-pull arrangement, means effective at times to apply to thecontrol grids of the said tubes 9. carrier current of a preselectedfrequency and a voice frequency current to create in a secondary windingof said output transformer the side bands of said carrier modulated byvoice frequencies, a

transmitting circuit, means including a filter-effective to pass apreselected one of said side bands only to couple said secondary windingto said transmitting circuit for transmission of such single side bandtelephone current, means effective at other times to apply to thecontrol grids of said tubes a signaling current of a single fre-' quencypreselected within the side band passed by said filter, a shunt circuithaving a predetermined resistance, and control means operable to openthe plate circuit of one of said tubes and to connect said shunt circuitacross that portion of said primary winding included in the platecircuit of the other tube for transmission of such signaling current ata predetermined energy level.

9. In transmitting apparatus for a communication system using suppressedcarrier transmission the combination comprising, a balanced modulatorincluding two electron tubes the plate circuits of which includerespective portions of an output transformer winding disposed in apush-pull arrangement and the grid circuit of which includes a firstresistor having its outside terminals connected one to each grid and itscenter terminal connected to the cathode of each tube through a secondresistor, a carrier current generator connected across said secondresistor, a microphone circuit including a microphone connected acrosssaid first resistor for modulating the carrier current by the voicefrequencies produced in said microphone and creating correspondingsuppressed carrier side band telephone currents in a secondary windingof said output transformer, a transmitting circuit, means including afilter effective to pass a selected one of said side band telephonecurrents only to couple said secondary winding to said transmittingcircuit for transmission of such single side band telephone current, ashunt circuit including a resistor; and a control relay effective whenpicked up to condition said generator to supply a'signaling current of asingle frequency preselected within the side band passed by said filter,to open the plate circuit of one of said tubes and to close said shuntcircuit across said primary winding for transmission of such v signalingcurrent at a predetermined energy level.

10. In transmitting apparatus for a communication system usingsuppressed carrier transmission. the combination comprising, a generatornormally adjusted to deliver a carrier current of a preselectedfrequency, a microphone, a modulator including two electron tubes Whosegrid circuits are coupled to said generator and to said microphone andthe plate circuits of which tubes include respective portions of anoutput transformer primary winding disposed in a push-pull arrangement,said modulator effective to create in a secondary winding of the outputtransformer the side bands of said carrier modulated by the voicefrequencies produced in said microphone with the carrier suppressed;control means effective at times to adjust said generator to deliver asignaling current of a frequency equal to said preselected frequencymodulated by a preselected voice frequency, to open the plate circuit ofone of said tubes and to connect a shunt resistance across the outsideterminals of said output transformer primary winding for creating insaid output transformer secondary winding signaling current because ofthe unequal portions of the active tube plate circuit current flowing inthe two portions of said primary winding, a transmitting circult, andcoupling means including a filter effective to pass only a preselectedone of said side bands to couple said transmitting circuit to saidsecondary winding.

LELAND D. WHII'ELOCK. RALPH K. CROOKS.

